Ensure that the response of cached server responses originates from the url used

While caching the answers of external servers is vital for harvester development, all of us have at least a few times run into the issue that our key parameter was unintentionally matching a response of another url.

The client should

• persistently store the url linked to a response from a server
• when discovering a response already exists for a certain key, check the url against the url used to achieve this response
• when the urls don't match, raise an Error